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Materials Basics > Chapter 8 - Reactor Pressure Vessel > Flashcards

Chapter 8 - Reactor Pressure Vessel Flashcards

1
Q

What is preferred, welding or forging?

A

forging

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2
Q

What is the yield strength range of steels used for RPV?

A

220 to 350 MPa

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3
Q

What is the ultimate tensile strength range of steels used for RPV?

A

350 to 450 MPa

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4
Q

What steel do they use for RPV forging?

A

SA508 which contains nickel, Mo, Cr and Mn

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5
Q

What are the four main welding techniques?

A
  • Shielded metal arc welding
  • gas tungsten arc welding
  • Metal inert gas (MIG) welding
  • submerged arc welding
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6
Q

What welding zones are sensitive to cracking?

A

Heat affected zone

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7
Q

How does the grain vary from base metal to weld metal?

A
  1. Base Metal
  2. Fine Grained HAZ
  3. Coarse Grain HAZ
  4. Penetration Zone
  5. Weld Metal
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8
Q

Why do the community move to forged RPV over welded?

A

because of the heat affected zones which can cause cracking , less pieces mean less breakage and higher strenghts.

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9
Q

What is the difference between welding and brazing?

A

welding is a technique that joins metals by melting the base metal and causing fusion, while brazing joins metals by melting and flowing a filler metal into the joint.

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10
Q

What is the differnce in the HAZ when using low and high hea tinput?

A

With low heat input the coarser grains are fewer and the HAZ is smaller. In high heat input the HAZ covers more area (is wider) and the coarser grains are more. The fusion zone also penetrates more.

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11
Q

How can we predict RPV embrittlement estimates in a real life reactor?

A

capsules are included with representative RPV steels located on the inside of the operating RPV (higher dose rate) to provide early embrittlement estimates and database

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12
Q

How do you test for embrittlement?

A

charpy v-notch

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13
Q

What is the charpy v notch test?

A

is a standardized high strain-rate test which determines the amount of energy absorbed by a material during fracture. Absorbed energy is a measure of the material’s notch toughness

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14
Q

How do we use surveillance samples in RPV?

A

These are used to get embrittlement data. The samples receive higher neutron doses and therefore we can observe the transition temperature shifts.

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15
Q

What is the usual temperature transition shift for a neutron dose of 1E20?

A

100 C degrees

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16
Q

What type of defects have a big impact on DBTT?

A

Impurities can have a large effect on DBTT

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17
Q

What is the effect of copper in the change in transition temperature? How big is the delta t?

A

as the concentration increases the transition temperature increases drastically and then plateaus.

The delta t is around 125 C

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18
Q

Why do we use nickel in terms of DBTT.

A

The transition temperature change is negligible allowing it to stay ductile at low temperatures and operate a high temperatures.

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19
Q

Why can Nickel accelerate the increase in transition temperature drastically?

A

Nickel coupled with increasing copper impurities can cause a shift in transition temperature of up to 300 degrees.

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20
Q

What is the operating temperature effect in transition temperature shifts? How big?

A

The higher the temperature the lower the transition temperatures.

At 305C = 10 to 20 delta T
At 270C = 20 to 60 delta T

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21
Q

Name the four steps of the embirttlement processes?

A
  1. Irradiation
  2. Ultra finescale micro/nanostructural evolution
  3. yield and flow stress elevation (hardening)
  4. shifts in transition toughness curves (embrittlement)
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22
Q

What are the 4 finescale hardening features?

A
  1. Cu rich precipitates
  2. stable matrix features (vacancy solute complexes)
  3. unstable matrix defects
  4. other alloy precipitates (p, c, N, Mo, Mn)
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23
Q

How do we determine if we need to impalement safety actions to protect against PTS fracture events?

A

By calculating the reference temperature. If the reference temperature at the EOL is higher than given 300 (for welds) then flux reduction program and safety analysis are necessary

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24
Q

What is the reference temperature a function of?

A
  • nickel and copper content
  • fluence
  • margin to account for uncertainties which is a function of welds and base metal.
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25
Q

What makes a material experience hardening in irradiated materials with impurities?

A

The precipitaes can cause hardening

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26
Q

What is a coherent precipitate?

A

A precipitate that is a continuation of the lattice structure of the solvent and has no phase or grain boundary.

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27
Q

What is the orowan mechanism?

A

When a dislocation bows around a precipitate therefor unpinning it and leaving a dislocation loop around said particles.

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28
Q

What is a davidnikov diagram?

A

rationalizes the link between radiations strengthening and increase in the ductile to brittle transition temperature.

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29
Q

Is there an issue with the NRC proposed method for predicitng temperature shifts? Explain.

A

Yes, the NRC method significalty under predicts the delta T at high flux*time due to eh compound effects of flux and other embrittlement variables.

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30
Q

What is the fracture toughness

A

asses the resitance of a material to crack propagation

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31
Q

Where is the stress located on a crack?

A

On the tip or flaw making it a non uniform stress state

32
Q

What is the issue with uniaxial tests with fractures? What is used to better predict the fracture toughness?

A

It assumes uniform stress and no flaws

Also, experiments on glass fibers that Griffith himself conducted suggested that the fracture stress increases as the fiber diameter decreases. Hence the uniaxial tensile strength, which had been used extensively to predict material failure before Griffith, could not be a specimen-independent material property. Griffith suggested that the low fracture strength observed in experiments, as well as the size-dependence of strength, was due to the presence of microscopic flaws in the bulk material.

33
Q

What does the parameter K mean int erms of griffits equation? What does it mean?

A

It is the critical fracture toughness. If this value is exceeded the crack will suddenly propagate

34
Q

What does the parameter K depend on?

A
  • The crack length
  • The geometry of the crack
  • the stress state in front of the crack tip
  • applied stress at which the crack propagates
35
Q

How do embrittlement of the vessel war occur during normal operation?

A

due to fast neutron damage

36
Q

What is the effect of fast neutron damage on hardness and on the fracture toughness?

A

The hardness increases therefore increasing embrittlement but reduciton in fracture toughnes (K)

37
Q

Is brittle fracture a problem in normal operation? Why?

A

No since the operating temperature is high at 300 C.

38
Q

What are the main stresses during normal operation on the RPV?

A

Is due only to residual stresses and the membrane (hoop) stress due to the internal pressure in the vessel.

39
Q

What is the hoop stress?

A

is a stress distribution with rotational symmetry; that is, which remains unchanged if the stressed object is rotated about some fixed axis.

Cylinder stress patterns include:

circumferential stress, or hoop stress, a normal stress in the tangential (azimuth) direction.

40
Q

What are the residual stresses?

A

Residual stresses are those stresses that remain in an object (in particular, in a welded component) even in the absence of external loading or thermal gradients. In some cases, residual stresses result in significant plastic deformation, leading to warping and distortion of an object.

41
Q

What does ECCS stand for?

A

emergency core cooling system

42
Q

What does the ECCS do?

A

It injects cold water due to a LOCA near the end of the vessel lifetime.

43
Q

What does LOCA stand for?

A

Loss of coolant accident

44
Q

Why is a LOCA dangerous even with ECCS kicking in?

A

Because the fracture toughnes is at it lowest due to irradiation embrittlement and therefore brittle failure is possible.

45
Q

What does rapid reduction in temperature from ECCS causes?

A

The inner wall temperature during ECCS creates tensile thermal stresses which add to existing stresses

46
Q

What is the thermal stress?

A

mechanical stress created by any change in temperature of a material. These stresses can lead to fracturing or plastic deformation depending on the other variables of heating, which include material types and constraints.[1] Temperature gradients, thermal expansion or contraction and thermal shocks are things that can lead to thermal stress. This type of stress is highly dependent on the thermal expansion coefficient which varies from material to material. In general, the greater the temperature change, the higher the level of stress that can occur. Thermal shock can result from a rapid change in temperature, resulting in cracking or shattering.

47
Q

What could happend due to shutdown and startups in terms of cracks?

A

Cracks and flaws on the inner wall of the vessel may have grown throught the lifetime due to fatigue.

48
Q

Will the crack definitely penetrate the entire vessel?

A

No, The crack may not penetrate the entire vessel wall because temperature, stress and irradiation dose change through the wall thickness. The crack will stop
when the applied K I reaches the crack arrest toughness K Ia

49
Q

What are the four manin ramedies for rpv embrittlement and PTS?

A
  1. annealing critical zones to remove radiation embrittlement
  2. change fuel management strategy (go to low leakage core)
  3. replace steel reflector elements at core periphery with more absrober materials (tungsten and titanium hydride)
  4. heat the ECCS water to avoid/reduce thermal shock
50
Q

What does PTS stand for?

A

pressurized thermal shock

51
Q

What is the initial licensed lifetime of an NPP in the USA?

A

30 to 40 years

52
Q

What is the main job of the RPV?

A

safely contain coolant water ate temperatures around 290 C and pressures of 7 MPa for BWR and 15 MPa for PWR

53
Q

What is the irradiation embrittlment characterized by?

A

the increase in a ductile to brittle transition temperature (DBTT)

54
Q

What are three improvments made to reduce RPV embrittlement?

A
  1. Lower trace impurity contents
  2. Reductions in neutron flux impinging the vessel
  3. Elimination of beltline welds
55
Q

What is a tipycal weight, height, diamater andthicknes of an RPV?

A

500 tonnes
14m high
4.5m in diameter
20cm wall thickness

56
Q

What is the fluence?

A

the neutron flux integrated over time (flux*time)

57
Q

What is the EOL fluence of a PWR?

A

1e23 to 3e23

58
Q

What does the symbol K_ic denote?

A

fracture toughness

59
Q

What are the four primary mechanisms processes for embrittlement?

A
  1. Generation of lattice defects forming single and small clusters of vacancies and SIAs
  2. Diffusion of primary defects leading to enhanced solute diffusion and formation of nanoscale defect-solute cluster complexes, solute clusters and distinct phases primarily copper rich precipitates
  3. Dislocation pinning and hardening by these nanofeatures
  4. Hardening-induced DBTT shifts
60
Q

The nanoscale defects produced form primary irradiation defects can be divided into three broad categories:

A
  1. Copper rich precipitates and catalyzed manganese-nickel rich precipitates
  2. unstable matrix defects that from in cascades
    3 stable matrix features that persists or grow under irradiation
61
Q

What is radiation enhanced diffusion? What is the main product in RPV?

A

a phenomena that enalbes solute diffusion due to the excess vacancy concetration. tHIS CAUSES THE FORMATION OF FINE SCALE COPPER RICH PRECIPITATES

62
Q

Why is copper an issue in RPV under irradiation?

A

Because of the relatively low solubility in an iron matrix (less than 0.01%) causing it to precipitate. Irradiation accelerates the precipitation causing precipitate hardening.

63
Q

What are CRP enriched with? What is the secondary effect?

A

Manganese and nickel and small amounts of phosphorous and silicon. Amplify the effect of copper hardening by increasing the volume of the precipitates.

64
Q

What are MNPs?

A

manganese nickel precipitates

65
Q

What is the interaction between MNP and CRP

A

they amplify the effect of hardening and embrittlement and in some cases MNP can replaced CRP by surrounding them (crp rich core).

66
Q

What does post irradiation annealing do?

A

It disolves the stable matrix features in the RPV INCLUDING CRP and MNP

67
Q

What temperatures does annealing happen for RPV?

A

375 TO 400 C

68
Q

What is one of the most important alloying impurities?

A

Copper

69
Q

What is the critical fracture toughness a funciton of?

A

the geometry factor, the applied stress at which crack propagates, the crack length

70
Q

What happens if the material experiences a force above the critical fracture toughness?

A

The crack will propagate.

71
Q

What is the fracture toughness units/

A

MPa/sqrt(m)

72
Q

Are small flaws and cracks in the inner wall something to wrry during normal operation? Why?

A

No, since at high temperatures the main stress is due to residual stress and the internal pressure in the vessel.

73
Q

What happens to the fracture toughness during irradiation? Can you keep reducing the Kic in a LOCA?

A

It goes down. Yes, after cold water is injected the Kic is at is lowest

74
Q

Is brittle failiure a problem in LOCA?

A

Yes, smal flaws cracks in the inner wall may have grown througut the lifetime due to fatigue during shutdown and starts

75
Q

Does Kic vary in the vessel? Why?

A

Yes, it varies since irradiation penetrates only a radial section of the RPV, there is an arrest fracture toughness that will stop a flaw from propagating any further

76
Q

What is the assumed initial flaw size for RPV

A

25% of the thickness

77
Q

What type of stresses go into the calculation for the critical fracture toughness?

A

the membrane hoop stress due to internal pressure of the vessel
residual stress due to fabrication and welding which is a function of the wall thickness
thermal stress due to thermal stresses from ECCS

Materials Basics (11 decks)

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